The role of the spin state of cobalt in determining the electrical and magnetic properties of cobalt oxides

Yi-Ying Chin^1*, Zhiwei Hu², Yuichi Shimakawa³, Junye Yang⁴, Youwen Long⁴, A. Tanaka⁵, Liu Hao Tjeng², Hong-Ji Lin⁶, Chien-Te Chen⁶, S. Agrestini², J. J. Li^7,8, X. X. Wang^7,8, K. Yamaura^7,8

¹Department of Physics, National Chung Cheng University, Chiayi, Taiwan
²Max Planck Institute for Chemical Physics of Solids, Dresden, Germany
³Institute for Chemical Research, Kyoto University, Kyoto, Japan
⁴Institute of Physics, Chinese Academy of Sciences, Beijing, China
⁵Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi-hiroshima, Japan
⁶National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁷National Institute for Materials Science, Ibaraki, Japan
⁸Department of Chemistry, Hokkaido University, Hokkaido, Japan

* Presenter:Yi-Ying Chin, email:yiyingchin@ccu.edu.tw

The competition between exchange energy and crystal field splitting stabilizes various spin states in correlated 3d transition metal oxide systems. This leads to the observed exceptional magnetic and electrical properties. A broad range of electric and magnetic properties characterizes A-site ordered perovskites of the AA'₃B₄O₁₂ type, attributed to interactions among B sites and between A' and B sites, which are influenced by the tilting of BO₆ octahedra. In CaCu₃Co₄O₁₂, replacing Ca with Y induces a metal-insulator transition, with YCu₃Co₄O₁₂ exhibiting an insulating state. On the other hand, the single-layer compound Sr₂CoO₄ exhibits both ferromagnetic and metallic behaviors, a combination not found in any other similar system. Understanding those properties requires detailed knowledge of the electronic structures of Co and Cu, for which X-ray absorption spectroscopy (XAS) has been utilized. Therefore, we performed XAS experiments on those compounds.
The Cu-L₃ XAS spectra of both CaCu₃Co₄O₁₂ and YCu₃Co₄O₁₂ confirm that Cu maintains a 3+ valence state in these compounds. Consequently, it is deduced that their electrical properties are primarily governed by the electronic structure of Co rather than that of Cu. Conversely, the spectral line shape and energy position of Co in YCu₃Co₄O₁₂ closely resembles those of EuCoO₃, indicating a low-spin (LS) Co3+ state in YCu₃Co₄O₁₂. In the LS Co3+ state with S=0, the t2g shell is filled, leading to the absence of an eg electron and the insulating nature of YCu₃Co₄O₁₂. Conversely, introducing Co4+ through chemical doping is associated with the metallic properties observed in CaCu₃Co₄O₁₂.
On the other hand, our integrated experimental and theoretical XAS studies indicate that the intermediate spin (IS) state of 4+ ions in Sr₂CoO₄ is stabilized by significant hybridization between Co 3d and O 2p states. This results in a ground state predominantly characterized by the ligand hole configuration 3d⁶L within the negative charge transfer region. Additionally, the IS state is further stabilized by the extended tetragonal distortion linked to the layered crystal structure of this compound, in contrast to the cubic SrCoO₃.

Keywords: Spin state, Valence state, X-ray absorption, Ferromagnetic metallic, Negative charge transfer